Negatively preloaded thrust unit

ABSTRACT

A pressure-responsive load cell comprises an output thrust member coaxially positioned within an externally pressurized bellows. The thrust member is attached to one end of the bellows, passes through and extends beyond the opposite end of the bellows. The portion of the thrust member within the bellows is a hollow cylinder in which a compression spring is located to provide a bias thrust towards the output end of the thrust member. In order to stabilize this inherently unstable configuration and to keep the assembly friction free, a thin, flat, narrow, very flexible metal guide is attached to the end of the bellows to which the thrust member is attached. To increase stability, a second such guide member may be employed. The second guide may be in the same plane as the first guide and separated therefrom by an angle of less than 180* to prevent positive biasing of two guides in one direction and to insure smooth movement of the guided assembly.

[451 May 30, 1972 United States Patent Sorteberg [54] N EGATIVELYPRELOADED THRUST Primary Examiner- Drayton E. Hoffman Attorney-Mauern,Ware and Davis UNIT [72] Inventor:

Johannes Sorteberg, Sorteberg Controls ABSTRACT Corp., 540 ConnecticutAvenue, South 7 claim, 2 Drawing mgm-s 3,289,933 l2/l966Sorteberg...........,..................235/200 IIIIIIII NEGATIVELYPRELOADED TI-IRUST UNIT BACKGROUND OF THE INVENTION ln a positivelypreloaded thrust cell, such as those shown in my U.S. Pat. Nos.2,643,055 and 2,918,214, a spring resists the outward movement andexertion of force by a thrust rod in response to increased air pressurein the cell. A negatively preloaded thrust unit is desirable inperforming certain calculations. In such a unit a spring would produce aforce tending to move the thrust rod outward from the cell incooperation with the air pressure supplied to the cell. When the thrusttransmitting member, or thrust rod abuts against a weighbeam, the forcetransmitted against the weighbeam comprises the sum of the force exertedby the spring and the force exerted by air pressure in the cell.

A negatively preloaded thrust unit of the present invention generallycomprises a coil spring located in axial relationship with the thrustrod. However, the spring is flexible and the axial relationship is not`rigid and the parts may-easily buckle or bend, especially when the coilspring is compressed. If the axial relationship is maintained by asurrounding guide sleeve, buckling of the spring may result in thespring or thrust rod touching or engaging the surrounding guide sleevethereby causing a frictional effect which is undesirable in theoperation of a weighbeam of balance or other related apparatus.

OBJECTS OF THE INVENTION lt is therefore a principal object of thepresent invention to provide a negatively preloaded power cell which isfree from any frictional effects.

lt is afurther object of the invention to provide a negatively preloadedpower cell for use in devices such as my patented weighbeam systems orrelated devices.

It is yet another object of the invention to provide an adjustablenegatively preloaded power cell with lthe above characteristics. l Otherobjects ofthe invention will in part be obvious and will in part appearhereinafter.

THE DRAWINGS For a fuller understanding of the nature and objectsof theinvention, reference should be had to the following detailed descriptiontaken in connection with the accompanying drawings in which:

FIG. l is a vertical-sectional view of an adjustable negativelypreloaded pressure-responsive load cell according to my invention; and,

FIG. 2 is a cross-sectional view taken along the lines 2-2 of FIG. l.

The same reference characters refer to the same elements throughout thetwo views of the drawing. l

The invention accordingly comprises the features of con struction,combination of elements, and arrangement of parts which will beexemplified in the construction hereinafter set forth, and the scope ofthe invention will be indicated in the v claims.

DETAILED DESCRIPTIONOF THE INVENTION An adjustable negatively preloadedthrust unit is generally indicated at 10 of FIG. l. This thrust unitcomprises first a base 1l constructed for mounting on a device such asmy patented weighbeam system, the receptive mounting portion of which isindicated at 12 of FIG. 1. A right circular cylindrical cap 14 is firmlyattached to the base 11 by inserting the lower edge 15 into the annulargroove 18 formed in the base l l and soldering or otherwise suitablysecuring the connection in an airtight manner. The lower edge 21 of abellows 20 is inserted in an annular groove 22 and also sealed to thebase 1l. The upper edge 23 of bellows 20 is sealed in a groove 24 formedin a bellows collar 26. A right cylindrical sleeve 28 is fitted incollar 26. Sleeve 28 extends downward from bellows collar 26 inside thebellows 20 and is sealed to a thrust rod collar 30. Sealed in the thrustrod collar 30 and extending through opening 13 formed in the base 1l isa thrust rod 32. All the above-mentioned seals are airtight and made bysoldering or the like.

ln the operation of the unit the lower end 33 of the thrust rod 32 abutsagainst a fulcrum pin 37 which is a part of a weighbeam system, aportion of which is indicated at 34. The weighbeam system 34 may be ofthe type disclosed in my above-mentioned patents. A coil spring 36 isfitted over and engages the stepped annular thrust rod collar 30. Theopposite end of this coil spring is similarly fitted over a collar 40formed on the closed end of bearing housing 41 which receives a ball 42and a shaft 43 abutting against the ball 42. The shaft 43 extendsdownward from a bolt 44. The bolt 44 has conventional screw threads 45which mate with the screw threads on a stud 46 which is firmly sealed tothe top of the cap 14. A sealing collar 47 having an annular groove 48containing an O ring 49 is also pressed and sealed onto rod 43 to sealthe space between the rod 43 and the recess in stud 46.

It will thus be seen that a sealed airtight chamber 50 is formedsurrounding bellows 20. A fluid is admitted through passage 54 in cap 14to pressurize or depressurize chamber 50. Chamber 52 communicates withthe interior of the weighbeam housing through passage 13 in base l l.The collar 30 and thrust rod 32 receive and transmit force in responseto the pressure differential between chambers 50 and 52. ln operationthe chamber 52 is normally maintained at atmospheric pressure whereaschamber 50 is pressurized according to the weighbeam air pressurebalancing system.

A force in addition to that exerted because of the air pressuredifferential between chambers 50 and 52 as described above is providedby coil spring 36. The spring is compressed by turning bolt 44 into stud46, thereby producing adjustable movement downward with respect to thetop of cap 14 of bolt 44, shaft 43, ball 42, and bearing housing 4Iagainst which spring 36 abuts. When compressed in this manner, spring 36exerts a force which is transmitted to the weighbeam 34 by collar 30 andthrust rod 32. The force is different for different positions ofthreaded bolt 44, and thus the force is adjustable.

Spring 36 is a flexible member of the adjustable forceproducing meanscomprising bolt 44, shaft 43, bearing 42, bearing housing 41, collar 30,and thrust rod, 32. The maximum downward force for any given position ofadjustable bolt 44 is transmitted to the weighbeam when the elements aremaintained in axial relationship because the spring is thereby mostfully compressed for that given position of bolt 44; however, the systemis inherently unstable in that condition and will buckle or bend unlesssupported. To maintain the accuracy necessary for successful operationof a weighbeam unit, the thrust means must not introduce anyunpredictable forces. An example of an unpredictable force producing aninaccurate output from the thrust unit would be to employ a tight sleevesimilar to cylindrical sleeve 28 as the axial relationship maintainingmember. In such an assembly the sleeve merely restrains the buckling toa minimum. There is a small amount of buckling within the sleeveresulting in contact between the sleeve and the spring, or the thrustrod collar, or both. Such contact with a rigid sleeve prevents bucklingand maintains alignment of the adjustable force producing members, butalso introduces a frictional effect into the thrust units output. Theundesirable frictional effect is the result of static and slidingfriction between the sleeve and the spring or thrust rod collar.

To eliminate this problem an elongated stud 60 is mounted on base l1neai the perimeter of cap 14. Secured to the top of stud 60 by a bolt 61or other suitable means is one end of a leaf spring 62. Leaf spring 62extends perpendicularly to the axis of the thrust unit and is secured toprevent angular motion thereof. It is also securely attached at itsopposite end to collar 26. The leaf spring serves to guide the assemblycomprising collar 26, spring 36, sleeve 28, collar 30, and thrust rod 32to maintain thrust rod 32 and spring 36 in axial relationship with theother members of the adjustable force-producing assembly. This preventsbending of the assembly and the accompanying undesirable frictionaleffect.

Stud 60 provides a stationary support for one end of leaf spring 62 atthe greatestvdistance from sleeve 28 within cap 14. The mountingposition is so chosen because the opposite end of leaf spring 62 movesin an arc. A greater radius of this arc is obtained by using the longestleaf spring which can be conveniently enclosed within cap 14. Thegreater radius results in an arc more nearly an approximation of astraight axis, and a greater range of up and down movement can beachieved without undesirable contact of the sleeve against other membersof the assembly. It may also be desirable to use more than one leafspring; however, if more than one is used, the second one should not bedeployed opposite the first to avoid any buckling or oil can snap-actioneffect. An effective deployment of a second spring 64 is shown in FIG. 2by dotted lines.

A negatively preloaded thrust unit of the type described herein having abellows with a Vtof l square inch effective area can produce thrustforces in the range of to 5 pounds for pressure differentials from 0 to20 pounds per square inch. ln such a unit, the axial movement of thethrust member in response to pressure differentials between the twochambers is less than 0.005 inches as applied to systems. Larger unitsmay be constructed which will produce greater thrust.

lt will thus be seen that the objects set forth above, among those madeapparent from the preceding description, are efciently attained and,since certain changes may be made in the above construction withoutdeparting from the scope of the invention, it is intended that allmatter contained in the above description or shown in the accompanyingdrawing shall be interpreted as illustrative and not in a limitingsense.

lt is also to be understood that the following claims are intended tocover all of the generic and specific features of the invention hereindescribed, and all statements of the scope of the invention which, as amatter of language, might be said to fall therebetween.

Having described my invention, what I claim as new and desire to secureby Letters Patent is:

l. A preloaded load cell comprising:

A. a support base;

B. a flexible pressure responsive bellows mounted at one end to thesupport base;

C. rigid axial mounting means attached at one end to the unmounted freeend of said bellows, the other end of the axial mounting means beingpositioned within and substantially coaxial with said bellows;

D. a thrust rod attached in axial alignment to said axial mounting meanssubstantially at the other end of said axial mounting means, wherein theaxial mounting means and the thrust rod attached thereto seal theunmounted free end of said pressure responsive bellows, and wherein thepressure difference between the inside and the outside of said pressureresponsive bellows causes a force to be applied to the thrust rod in agiven axial direction;

E. a spring substantially within and coaxial with said axial mountingmeans and adjustably compressed between spring mounting means attachedto the base and the portion of the axial mounting means to which thethrust rods is attached for applying force to said thrust rod in thegiven axial direction; and

F. an axial guide fixed to the axial mounting means near the unmountedfree end of said bellows for limiting the radial motion of said axialmounting means transverse to said axial direction, thereby maintainingthe axial alignment of the spring, axial mounting means, and thrust rod.

2. A load cell as defined in claim l, wherein said axial guide has muchgreater stiffness in a circumferential direction than in said axialdirection.

3. A load cell as defmed in claim 2, wherein said axial guide comprisesa radially disposed thin, flat, narrow piece.

4. A load cell as dened in claim 2, wherein said spring is a coilspring. l

5. A preloaded load cell comprising:

A. a support base;

B. a flexible pressure responsive bellows mounted at one end to saidsupport base and closed at its other end; C. a thrust rod axiallyaligned within said bellows and attached to the closed end of saidbellows wherein the pressure difference between the inside and theoutside of said pressure responsive bellows causes a force to be appliedto the thrust rod in a given axial direction;

D. a spring compressed between spring mounting means attached to saidbase and the closed end of said bellows for applying force to saidthrust rod and bellows in the given axial direction; and

E. an axial guide member rigidly attached to the closed end of saidbellows and rigidly attached to mounting means attached to said base forlimiting the radial motion of said bellows, rod and spring transverse tosaid axial direction, thereby maintaining the axial alignment of thespring, bellows and thrust rod. K

6. A preloaded load cell as defined in claim 5 wherein there is a pairof said guide members, and the points of attachments of said guidemembers to said base and to said thrust rod do not cover an anglegreater than 7. A preloaded load cell as dened in claim 5 wherein saidaxial guide member is an elongated leaf spring mounted orthogonally tothe axial relationship to be maintained.

1. A preloaded load cell comprising: A. a support base; B. a flexiblepressure responsive bellows mounted at one end to the support base; C.rigid axial mounting means attached at one end to the unmounted free endof said bellows, the other end of the aXial mounting means beingpositioned within and substantially coaxial with said bellows; D. athrust rod attached in axial alignment to said axial mounting meanssubstantially at the other end of said axial mounting means, wherein theaxial mounting means and the thrust rod attached thereto seal theunmounted free end of said pressure responsive bellows, and wherein thepressure difference between the inside and the outside of said pressureresponsive bellows causes a force to be applied to the thrust rod in agiven axial direction; E. a spring substantially within and coaxial withsaid axial mounting means and adjustably compressed between springmounting means attached to the base and the portion of the axialmounting means to which the thrust rods is attached for applying forceto said thrust rod in the given axial direction; and F. an axial guidefixed to the axial mounting means near the unmounted free end of saidbellows for limiting the radial motion of said axial mounting meanstransverse to said axial direction, thereby maintaining the axialalignment of the spring, axial mounting means, and thrust rod.
 2. A loadcell as defined in claim 1, wherein said axial guide has much greaterstiffness in a circumferential direction than in said axial direction.3. A load cell as defined in claim 2, wherein said axial guide comprisesa radially disposed thin, flat, narrow piece.
 4. A load cell as definedin claim 2, wherein said spring is a coil spring.
 5. A preloaded loadcell comprising: A. a support base; B. a flexible pressure responsivebellows mounted at one end to said support base and closed at its otherend; C. a thrust rod axially aligned within said bellows and attached tothe closed end of said bellows wherein the pressure difference betweenthe inside and the outside of said pressure responsive bellows causes aforce to be applied to the thrust rod in a given axial direction; D. aspring compressed between spring mounting means attached to said baseand the closed end of said bellows for applying force to said thrust rodand bellows in the given axial direction; and E. an axial guide memberrigidly attached to the closed end of said bellows and rigidly attachedto mounting means attached to said base for limiting the radial motionof said bellows, rod and spring transverse to said axial direction,thereby maintaining the axial alignment of the spring, bellows andthrust rod.
 6. A preloaded load cell as defined in claim 5 wherein thereis a pair of said guide members, and the points of attachments of saidguide members to said base and to said thrust rod do not cover an anglegreater than 180*.
 7. A preloaded load cell as defined in claim 5wherein said axial guide member is an elongated leaf spring mountedorthogonally to the axial relationship to be maintained.